This invention relates to a mobile satellite communication method and a mobile satellite communication system for carrying out communications via a communications satellite between an earth station and a mobile station with an automatic tracking type antenna.
In a satellite communications system, multiplex techniques called multiple accesses are used in order to effectively utilize assigned frequencies and to decrease transmission power up to now. Among the multiple accesses, a frequency division multiple access (FDMA) system, which assigns frequencies for a satellite transponder, is widely used in the satellite communications system presently. This is because the FDMA system can apply technique established in a ground communication, has a low initial cost, and can be relatively easily realized.
However, the FDMA system is disadvantageous in that throughput drastically decrease when the number of channel connection increases. More specifically, when the number of channel connection increases, transmission power in the satellite transponder increases and an amplifier in the transponder may operate at an operating point near to a saturation point thereof. Therefore, intermodulation accompanied with waveform distortion occurs and interference between channels occurs. As a result, quality of a communication path decreases, it is difficult to use the communication path, and throughput drastically decreases. Accordingly, in the satellite communication system of the FDMA system, use is made of carrier activation in order to avoid these problems.
For example, in a case of a satellite communications system offering a transmission service of speech, carrier activation is described by G. Maral, et al. in a book under the title of "SATELLITE COMMUNICATIONS SYSTEM 2nd Edition", John Wiley & Sons, Inc. 1993 or is described by Vijay K. Bhargava, et al., in a book under the title of "DIGITAL COMMUNICATIONS BY SATELLITE", John Wiley & Sons, Inc. 1981. According to these books, as the carrier activation, voice activation is carried out which detects a speech and transmits a carrier signal during a time duration when the speech is detected in a telephone call. In the voice activation, inasmuch as the carrier signal is not transmitted during a pause time duration in the telephone call when the voice is not detect, it is possible to decrease an average transmission power in the satellite and to ensure a channel capacity with intermodulation prevented.
FIGS. 6A and 6B are time charts for showing signals in a case of carrying out the voice activation. FIG. 6A shows a speech signal while FIG. 6B shows a transmission signal. During a time duration Ti when the speech signal is interrupted, sending of the transmission signal (a carrier signal) is basically stopped.
Such carrier activation is applied not only to a case of transmitting a speech signal but also to a case of transmitting a data signal for use in a facsimile equipment or a computer and transmission of the carrier signal is stopped when the data signal to be transmitted is not supplied.
In addition, in the satellite communications system carrying out the carrier activation, as shown in FIG. 6B, a burst carrier signal 102 is periodically transmitted at predetermined time intervals in order to maintain a channel connection state between an earth station and a mobile station when the time duration of a transmission stop is not less than a predetermined time duration with no information signal (no speech signal or no data signal to be transmitted) supplied. Such a burst carrier signal is call a maintaining burst signal.
On the other hand, when the mobile station uses an automatic tracking type antenna as a mobile antenna, the mobile station normally receives a signal from the satellite and carries out a tracking control on the basis of a level of the received signal so as to turn a direction of the mobile antenna toward the satellite. Accordingly, such a mobile station must carry out the tracking control using only the maintaining burst signal during a time duration when the carrier signal from the earth station is interrupted due to the carrier activation. It is difficult to control accurately when a bad condition is piled up.
Particularly in a case of a mobile satellite communications system, the received signal may be not received in stable due to a shadowing which occurs because variation of a transmission condition peculiar to a mobile satellite communication. It is increasingly difficult to carry out the tracking control in such a state. In addition, it is supposed in a mobile station loaded into a vehicle loaded with the automatic tracking type antenna that the mobile station rapidly turns in its moving direction. Under the circumstances, it is impossible to carry out the tracking control using only reception of the maintaining burst signal during operation of the carrier activation. This may cause a case where it is difficult only to maintain a received synchronization.
Therefore, in the mobile satellite communications system, it results in remarkably injuring reliability in a communication path in a case of carrying out the carrier activation.
As a method of avoid this problem, there is a method of always transmitting a tracking exclusive signal from the satellite, and of monitoring a signal level of the tracking exclusive signal in the mobile station. However, inasmuch as this method requires hardware for receiving the tracking exclusive signal and has a large scale of hardware as a whole, this method is unsuitable for the mobile station which requires high mobility.